1. Field of the Invention
The present invention is related to satellite attitude control and in particular to a method and apparatus for balancing secular satellite torques by tilting of the satellite solar panels toward or away from the sun as a means of balancing the environmental disturbance torques applied to the satellite.
2. Description of Related Art
Orbiting satellite are subject to various environmental disturbance torques. For satellites in a geosynchronous orbit, the environmental disturbance torque is primarily due to solar radiation pressure. Photoelectric power generation onboard a satellite requires the exposure of large surface areas to the sun. Nominally, these areas are symmetrically disposed with respect to the satellite center of mass so that the moment of the resultant solar force is zero. However, any misalignments or asymmetries in these areas may result in the creation of a relatively large solar torque. Other environmental effects such as the satellite magnetic dipole interaction with the earth's magnetic field or the torque due to the gravity gradient across an asymmetrical mass distribution are typically made negligible by design. In some cases, they may be used to provide additional control torque to counteract the effects of unwanted environmental disturbance torques.
The net solar pressure torque experienced by a satellite in geosynchronous orbit varies over the course of a day, relative to an inertial reference system, since the central body typically rotates at an orbit rate with respect to the sun, keeping one face of the satellite pointed at the earth. However, the solar torque generated by an asymmetric disposition of the solar panels is essentially constant over a day with respect to an inertial reference system, since the panels maintain a fixed orientation to the sun.
The disturbance torque may be expressed as the sum of two terms: the diurnal average and the remainder after subtracting the average value. The average inertial torque is referred to as the secular torque while the remainder, by definition, is the periodic torque. This distinction is significant since only the secular torque causes a net change in the angular momentum of the system. The periodic torque causes diurnal variation in the angular momentum but the net change which results is zero. If the satellite attitude control system is properly designed, the change in angular momentum due to environmental disturbance torques is absorbed by the momentum management system of the satellite which must be capable of handling both the secular and periodic torques. When the angular momentum storage capacity of the momentum management system is reached, the stored angular momentum must be "dumped", typically by using the reaction control system to restore the net system angular momentum to zero or a desired momentum (bias) state. Because of the degradation in pointing accuracy during thruster activity, the angular momentum storage capacity of the satellite is typically sized to provide an interval between angular momentum dumps which is the same as the required stationkeeping interval.
In general, the secular torque may be resolved into three orthogonal inertia components with reference to the sun: overturning, windmill and pitch. The overturning and windmill components are in the orbit plane, with the windmill component being along the projection of the sun line and the overturning component of the torque being perpendicular to the projection of the sun line. The pitch component is along the orbit normal as shown in FIG. 1.
Several factors result in the creation of solar pressure disturbance torques on a satellite such as: a) solar panel alignment and variation in solar cell reflectivity; b) solar panel deformation due to dry-out and thermal expansion; c) antenna reflector and satellite body geometry; and d) differences in the satellite center of mass and center of environmental force application. The uncertainty in defining contributions from the sources is large and therefore, satellites typically require powerful attitude control torque systems such as magnetic torque systems, large momentum and/or reaction wheels, large area variations in solar sailing systems, etc.
One approach to producing a counteracting overturning torque on a satellite is to vary the distances by which the solar panels extend from the satellite body. Thus, the forces produced by the solar radiation pressure on the panels act upon the satellite over two lever brackets of different lengths and thereby produces an overturning torque on the satellite. The same effect can be achieved by making the solar panels foldable and unfoldable, like an accordion, to vary the area of the solar panel and also the location of the area center relative to the satellite center of mass. To move the solar panels in or out from the satellite body however, requires a relatively complex mechanism.
Another approach to counteracting the secular overturning torque is to rotate one of the solar panels about its sun tracking axis relative to the opposite solar panel to effect the solar panel configuration and thus producing a counteracting overturning torque acting on the satellite. Such a method however produces a windmill torque which must be compensated for by either the satellite attitude control system or by alternatingly producing negative and positive windmill torques by rotating the solar panels in the opposite direction. Since the secular overturning torque is a constant value over the course of a day, it is necessary to perform a compensating maneuver periodically throughout the day to maintain proper attitude control.
It is an object of this invention to overcome the difficulties with the known methods for producing a torque to counter disturbance torques.
It is another object of the invention to compensate for the secular overturning torque with a balancing maneuver to eliminate continued torque compensating maneuvers.